I would test the power supply to the HCV. Turn ignition on and check the voltage when set to hot and to cold. If it is the same it is the heater control switch that needs replacing. I can see you tried changing it but it may have been a duff one.

Personally I would just sign it on his behalf. There would only be an issue if your Brother decided that he didn't want it sold and challenged the sale himself saying he hadn't signed it.
I've sold a car to Trade and signed it on behalf of my Daughter and the Traders weren't at all bothered. DVLA do not check the signatures on the V5's as far as I can find.
Also in the old days of applying for car tax at a Post Office and having to sign and send off the V5 at the same time if you had just bought the car, you were allowed to sign it in your own name and then in brackets write your relationship to the new owner.

I've had the same symptoms on numerous cars I have bought and its only needed the pads changing. If they aren't bedded in properly for a few hundred miles by avoiding heavy braking they will cause the vibrations when braking from high speeds.

If anyone needs a copy please PM me with your email address and I will send it to you. Not sure the FX was in a Mk2 but was deffo in the Mk 2.5.
I'm away at the moment but will send it out on Monday as its on my main computer at home.
Cheers.Bruce

Cant blame the truckers. Many times when visiting my Mum in Addenbrookes I have spent 25 to 30 miles at 50 in the outside lane. You can see half a mile in front a lorry at the front of the queue. By the time that one gets over another one has pulled into the line and so on. Ive been stuck directly behind one for 7 or 8 miles while it drives along side another lorry. Its the fact that it is a busy haulage route with only two lanes that is the issue. Not the truckers.

There is only a 4.5% difference between the Fiesta and top spot. I don't know the size of the survey but the Fiesta numbers are likely to be more highly represented in comparison to the others and therefore the results could be slightly distorted.

The heating issue is the heater control valve. They cost about £40 and take around 20 minutes to half hour to fit.
The Ka fans are noisy especially on the older cars.
The coolant leak is likely to be the thermostat housing which is top right side of the engine as you are facing it. You will need to take the battery out to get to it. These are plastic and warp over time. One of the pipes from this goes to the heater control valve which is in the centre of the bulkhead.
If its not leaking from there (which it probably is) sometimes the expansion tank splits causing a leak under pressure.
The 1.6 engine is not very economical. We had the SportKa which has the same engine. The best we ever managed was 32 mpg.
I cant remember ever seeing a Streetka with a hard top. Only in photos.